diff --git a/test/test_orm_index.rb b/test/test_orm_index.rb
new file mode 100644
index 0000000..dfebb66
--- /dev/null
+++ b/test/test_orm_index.rb
@@ -0,0 +1,160 @@
+require 'minitest/autorun'
+require 'prism'
+require 'tmpdir'
+
+class OrmIndexStoreTest < Minitest::Test
+  def setup
+    @dir = Dir.mktmpdir('prism-index-store')
+    @store = Prism::Orm::Store.new(File.join(@dir, 'test.pstore'))
+    @store.create_table(:users, %w[id email age])
+  end
+
+  def teardown
+    FileUtils.remove_entry(@dir)
+  end
+
+  def test_add_index_returns_a_default_name_and_lists_it
+    name = @store.add_index(:users, :email)
+    assert_equal 'index_users_on_email', name
+    assert_equal [{ name: 'index_users_on_email', columns: ['email'], unique: false }], @store.indexes(:users)
+  end
+
+  def test_add_index_accepts_an_explicit_name
+    name = @store.add_index(:users, :email, name: 'idx_email')
+    assert_equal 'idx_email', name
+  end
+
+  def test_add_index_twice_with_the_same_name_raises
+    @store.add_index(:users, :email)
+    assert_raises(Prism::Orm::Store::IndexExists) { @store.add_index(:users, :email) }
+  end
+
+  def test_add_index_backfills_existing_rows
+    @store.insert(:users, email: 'a@x.com', age: 20)
+    @store.insert(:users, email: 'b@x.com', age: 30)
+    @store.add_index(:users, :email)
+
+    matches = @store.lookup_by_index(:users, { 'email' => 'a@x.com' })
+    assert_equal ['a@x.com'], matches.map { |row| row['email'] }
+  end
+
+  def test_add_unique_index_over_existing_duplicates_raises_and_creates_nothing
+    @store.insert(:users, email: 'dup@x.com', age: 20)
+    @store.insert(:users, email: 'dup@x.com', age: 30)
+
+    assert_raises(Prism::Orm::Store::UniqueConstraintViolation) do
+      @store.add_index(:users, :email, unique: true)
+    end
+    assert_equal [], @store.indexes(:users)
+  end
+
+  def test_remove_index_by_name
+    @store.add_index(:users, :email, name: 'idx_email')
+    @store.remove_index(:users, 'idx_email')
+    assert_equal [], @store.indexes(:users)
+  end
+
+  def test_remove_index_by_column_list
+    @store.add_index(:users, :email)
+    @store.remove_index(:users, :email)
+    assert_equal [], @store.indexes(:users)
+  end
+
+  def test_remove_unknown_index_raises
+    assert_raises(Prism::Orm::Store::UnknownIndex) { @store.remove_index(:users, :email) }
+  end
+
+  def test_lookup_by_index_returns_nil_without_a_covering_index
+    @store.insert(:users, email: 'a@x.com', age: 20)
+    assert_nil @store.lookup_by_index(:users, { 'email' => 'a@x.com' })
+  end
+
+  def test_lookup_by_index_returns_empty_array_for_no_match
+    @store.add_index(:users, :email)
+    @store.insert(:users, email: 'a@x.com', age: 20)
+    assert_equal [], @store.lookup_by_index(:users, { 'email' => 'nobody@x.com' })
+  end
+
+  def test_composite_index_requires_every_column_present_in_the_lookup
+    @store.add_index(:users, [:email, :age])
+    @store.insert(:users, email: 'a@x.com', age: 20)
+
+    assert_nil @store.lookup_by_index(:users, { 'email' => 'a@x.com' }),
+               'a lookup missing one of the composite columns should not use this index'
+    matches = @store.lookup_by_index(:users, { 'email' => 'a@x.com', 'age' => 20 })
+    assert_equal 1, matches.size
+  end
+
+  def test_lookup_by_index_prefers_the_most_specific_covering_index
+    @store.add_index(:users, :email)
+    @store.add_index(:users, [:email, :age])
+    @store.insert(:users, email: 'a@x.com', age: 20)
+    @store.insert(:users, email: 'a@x.com', age: 99)
+
+    matches = @store.lookup_by_index(:users, { 'email' => 'a@x.com', 'age' => 20 })
+    assert_equal [20], matches.map { |row| row['age'] }
+  end
+
+  def test_unique_index_rejects_a_duplicate_insert
+    @store.add_index(:users, :email, unique: true)
+    @store.insert(:users, email: 'a@x.com', age: 20)
+
+    assert_raises(Prism::Orm::Store::UniqueConstraintViolation) do
+      @store.insert(:users, email: 'a@x.com', age: 30)
+    end
+    assert_equal 1, @store.count(:users), 'the rejected insert must not have partially applied'
+  end
+
+  def test_unique_index_rejects_an_update_that_collides_with_another_row
+    @store.add_index(:users, :email, unique: true)
+    id1 = @store.insert(:users, email: 'a@x.com', age: 20)
+    id2 = @store.insert(:users, email: 'b@x.com', age: 30)
+
+    assert_raises(Prism::Orm::Store::UniqueConstraintViolation) do
+      @store.update(:users, id2, email: 'a@x.com')
+    end
+    assert_equal 'b@x.com', @store.find(:users, id2)['email'], 'the rejected update must not have partially applied'
+    assert_equal 'a@x.com', @store.find(:users, id1)['email']
+  end
+
+  def test_unique_index_allows_updating_a_row_to_its_own_current_value
+    @store.add_index(:users, :email, unique: true)
+    id = @store.insert(:users, email: 'a@x.com', age: 20)
+    @store.update(:users, id, age: 21)
+    assert_equal 21, @store.find(:users, id)['age']
+  end
+
+  def test_index_tracks_updates_old_key_freed_new_key_claimed
+    @store.add_index(:users, :email)
+    id = @store.insert(:users, email: 'a@x.com', age: 20)
+    @store.update(:users, id, email: 'b@x.com')
+
+    assert_equal [], @store.lookup_by_index(:users, { 'email' => 'a@x.com' })
+    assert_equal [id], @store.lookup_by_index(:users, { 'email' => 'b@x.com' }).map { |row| row['id'] }
+  end
+
+  def test_index_forgets_deleted_rows
+    @store.add_index(:users, :email)
+    id = @store.insert(:users, email: 'a@x.com', age: 20)
+    @store.delete(:users, id)
+    assert_equal [], @store.lookup_by_index(:users, { 'email' => 'a@x.com' })
+  end
+
+  def test_deleting_a_nonexistent_id_is_still_a_silent_no_op
+    @store.add_index(:users, :email)
+    @store.delete(:users, 999)
+  end
+end
+
+# add_column/remove_column must keep any index that covers the affected
+# column in sync - see the comments on Store#add_column/#remove_column for
+# why a stale map here is a silent-wrong-results bug (Relation#rows trusts
+# a non-nil #lookup_by_index result as definitive), not just a missed
+# optimization.
+class OrmIndexColumnMigrationTest < Minitest::Test
+  def setup
+    @dir = Dir.mktmpdir('prism-index-column-migration')
+    @store = Prism::Orm::Store.new(File.join(@dir, 'test.pstore'))
+    @store.create_table(:posts, %w[id title])
+  end
+